The effect of varying perfusion pressure on the adequacy and distribution of coronary flow was studied in normothermic and hypothermic beating hearts. At 37 degrees C., lowering perfusion pressure from 100 to 50 mm. Hg did not change oxygen uptake or total or regional coronary flow or cause biochemical or histochemical ischemia. Vasodilator reserve capacity, however, was expended in order to maintain adequate subendocardial perfusion. At 28 degrees C., myocardial oxygen uptake per minute fell but oxygen consumption per beat rose significantly. Reduction of perfusion pressure to 50 mm. Hg caused a 44 percent (p less than 0.01) reduction in subendocardial flow and resulted in redistribution of flow away from the subendocardium (endocardial/epicardial flow ratio fell from 1.25 to 1.0). Ischemia was evident from intracavitary electrocardiogram, abnormal glycolysis, and histochemical staining. These studies show that during normothermia the coronary arteries dilate to provide adequate coronary flow when perfusion pressure is reduced. In contrast, compensatory vasodilatation is inadequate in hypothermic hearts and ischemia occurs at low perfusion pressures.